Question: Find the value of $\cos ^{-1}\left(\cos \frac{13 \pi}{6}\right)$
Solution:
$\cos ^{-1}\left(\cos \frac{13 \pi}{6}\right)=\cos ^{-1}\left[\cos \left(2 \pi+\frac{\pi}{6}\right)\right]$
$=\cos ^{-1}\left[\cos \left(\frac{\pi}{6}\right)\right]$
$=\frac{\pi}{6}$
